2017 Forecast: Summer Hypoxic Zone Size Northern Gulf of Mexico

Each year a hypoxic water mass with oxygen concentrations ≤ 2 mg l-1 forms in bottom
waters of the northern Gulf of Mexico continental shelf. The low oxygen conditions threaten
living resources including humans that depend on fish, shrimp and crabs. Nutrients from the
Mississippi River watershed, particularly nitrogen and phosphorus, fertilize the Gulf’s surface
waters to create excessive amounts of algal biomass, whose decomposition in the bottom layer
leads to oxygen distress and even organism death in the Gulf’s richest waters. Various models
use the May nitrogen load of the Mississippi River as the main driving force to predict the size of
this hypoxic zone in late July. This prediction is based on one of these models.

The June 2017 forecast of the size of the hypoxic zone in the northern Gulf of Mexico for
late July 2017 is that it will cover 26,131 km2 (10,089 mi2) of the bottom of the continental shelf
off Louisiana and Texas. The 95% confidence interval is that it will be between 23,648 and
28,551 km2 (9,131 and 9,131 mi2). This estimate is based on the assumption that there are no
significant tropical storms occurring in the two weeks before the monitoring cruise, or during the
cruise. If a storm does occur, then the size of the zone is predicted to be 70% of the predicted
size without the storm, equivalent to 17,250 km2 (6,660 mi2).

The predicted hypoxic area is about the area of Vermont (24,901 km2) and 93% larger
than the average of 13,536 km2 (all years, including years with storms). If the area of hypoxia
becomes as large as predicted, it will equal about five times the size of the goal of the Hypoxia
Action Plan (i.e., less than 5,000 km2). Efforts to reduce the nitrate loading have not been
successful.

Caveats: 1) This prediction discounts the effect of large storm events that temporarily
disrupt the physical and biological system attributes promoting the formation of the low oxygen
zone in bottom waters; 2) The potential space on the shelf where hypoxia occurs is limited by the
bathymetry; 3) The predictions assume that there will be no abrupt changes in discharge from
now through July; and 4) Unusual weather patterns affecting coastal winds, as experienced in
2009 and 2011, may skew the prediction to be lower.